Plant Reproduction: Sexual Reproduction

General Reproduction

• Process by which living organisms produce offspring.
• Continuation of species is ensured by this process.

Reproduction in Plants

• New members are produced through this process, which ensures the continuation of life.
• Forms include sexual and asexual.

Sexual Reproduction in Plants

• Progeny is produced by the fusion of haploid cells called gametes from two organisms (male and female).
• Gametes are produced by meiosis, and their fusion creates a diploid zygote.
• Syngamy is the union of gametes, called Fertilization.
• Reproductive organs are located in the flowers in flowering plants.
• Flower: Specialized shoot with limited growth.
• Parts are arranged in whorls on a receptacle (flattened top of flower stalk or pedicel).
• Arrangement consists of:
  • Calyx (sepals): Covers and protects inner whorls during bud stage.
  • Corolla (petals): Brightly colored for pollinator attraction.
  • Androecium (stamen): Male organ with stalk (filament), head (anther), and pollen grains.
  • Gynoecium (pistil): Female organ with style, stigma, ovary, and ovule; ovules contain an egg cell.

Asexual Reproduction in Plants

• Does not involve gametes or participation of two organisms but one parent gives rise to the offspring.
• Common in plants; accomplished through:
  • Fission
  • Fragmentation
  • Spore formation
  • Vegetative propagation
• Spore formation:
  • Unicellular bodies formed by cell division in a parent organism.
  • Germinate directly or indirectly under favorable conditions into new individuals when dispersed.
  • Spores have thick resistant walls that enable them to survive unfavourable conditions.
  • Fungi are well noted for air borne spore production and spread rapidly on substrates.
• Fission:
  • Organism divides into two (binary fission) or more (multiple fission) equal sized parts.
  • Binary fission is characteristic of bacteria.
• Vegetative propagation:
  • Plant parts become detached and develop into self-supporting individuals.
  • Detached stems and leaves produce roots and grow into new individuals.
  • Rhizomes propagate plants like spear grass, ginger, and carpet grass.
  • Detached leaves of Begonia and Bryophyllum can produce roots.
  • Cassava stem cuttings are examples.
• Fragmentation:
  • Organism breaks into two or more pieces, each regenerating into a new individual.
  • Common in filamentous algae; fungi reproduce by fragmentation of hyphae.

Reproduction in Animals

• Offspring is produced through Asexual or Sexual reproduction, each having advantages and disadvantages.

Asexual Reproduction in Animals

• Occurs in many eukaryotic, single-celled and multi-celled organisms.
• Advantages: Produces offspring genetically identical to the parent, single individual can produce offspring asexually, and large numbers of offspring can be produced quickly.
• Forms include:
  • Fission/Binary Fission:
    • Invertebrate, multi-cellular organisms reproduce by splitting into two parts.
    • Missing parts regenerate.
    • Seen in Planarians (Flatworms): Separate into head and tail regions, regenerating the missing half.
  • Budding:
    • A part of the body outgrows, leading to the separation of the “bud” from the original organism and the formation of two individuals, one smaller than the other.
    • Common in invertebrates like Hydras and Corals.
    • In Hydra, the bud develops into an adult and breaks away from the main body.
  • Fragmentation:
    • An organism breaks into parts followed by regeneration.
    • Separate individual regrows from each part if the animal is capable of fragmentation and the parts are big enough.
    • May occur through accidental damage, damage from predators, or as a natural form of reproduction.
    • Observed in Sponges and Echinoderms;.
    • A new individual regenerates from a broken arm and a piece of the central disc in some sea stars.
  • Parthenogenesis:
    • An egg develops into an individual without being fertilized.
    • Resulting offspring can be haploid or diploid.
    • Occurs in invertebrates such as water fleas, aphids, stick insects, ants, wasps, and bees.
    • Ants, bees, and wasps produce haploid males (drones).
    • Some vertebrates such as certain reptiles, amphibians, and fish also reproduce through parthenogenesis.

Sexual Reproduction in Animals

• Creates a new organism by combining the genetic materials of two organisms.
• Begins with meiosis, a specialized type of cell division.
• Each parent contributes half of the offspring's genetic makeup by creating haploid gametes.
• Complex life cycle where diploid mother cells divide to produce haploid cells (gametes) through meiosis.
• Fusion of a sperm and an egg called Fertilization.
• Fertilization is :
  • Internal (inside)
  • External (outside)
  • External:
    • Typically in aquatic environments where eggs and sperm are released into the water and the fertilization takes place.
  • Internal:
    • Often in terrestrial animals, sometimes in aquatic animals.
    • Occurs by male depositing sperm in the female during mating.
    • Can occur by male depositing sperm in something protective in the environment, which by female picks up to deposit the sperm in her reprodutive tract.
• Following internal fertilization, There are three ways offspring are produced
  • i. Oviparous: The fertilized egg is laid outside the female body, usually enclosed in a hard shell.
    • Seen in all birds, some bony fishes, reptiles, amphibians, and a few mammals (platypus and echidna).
  • ii. Ovoviparous: Fertilized eggs are retained in the female's body until they hatch inside her.
    • She lays the eggs right before they hatch helps protect the eggs, occurring in sharks, lizards, snakes, and some invertebrate animals.
  • iii. Viviparous: Young usually born alive and obtain nourishment from the female mammary gland, being born in varying states of maturity.
    • Occurs in most mammals, some cartilaginous fish, and a few reptiles.

Hereditary

• Study of mechanisms of transmission of characters from parents to offspring.
• Hereditary is the transmission and expression of characteristics or traits in an organism from parents to offspring.
• Chromosome refers to the basic unit of hereditary.
• Only the traits that constitute the genetic makeup of the parents can be transmitted and expressed in the offspring.
• Traits include colour of the skin, colour of eyes, colour of the hairs and hair texture, size of body stature, shape of the head , shape of the ears, shape of the month, lips, shape of the nose, length of the hands and legs and neck , Blood grouping, baldness, tongue rolling, Transmissible traits in plants: colour and shape of the leaves, shoot , seed size and shape, colour of the flowers, size of the fruit and pigmentation.

Mendelian Laws of Genetics

• Gregor Mendel (1822-1884)
• Monk in Austria and Father of Genetics who's work formed the foundation for scientific study of heredity and variation.
• Performed several experiments on how hereditary characters were transmitted from generation to generation working with Garden Pea (Pisum sativum) which are usually self-pollinating and he could pollinate these plants by himself and they have a very short life span than animals and some other plants.
• Mendel's early experiments crossed a pure bred short plant with a pure bred tall plant. All the seeds produced tall plants and the short character was overshadowed or suppressed by the tall character.
• Dominant refers to the tall character which is said to be Dominant over the short character whose influence has been suppressed and which is said to be Recessive.
• First Filial Generation (F1) refers to the plants that resulting from a cross between pure bred tall and pure bred short plants.
• Mendel discovered two fundamental laws (the law of segregation of genes and the law of independent assortment of genes) on which the modern theory of heredity is based.
• First law - Law of segregation: genes are responsible for the development of individual and that they are independently transmitted from one generation to another without undergoing any alteration.
• Second Law -Law of independence assortment: When more than one factor are considered, each character behaves as a separate unit and is inherited independently of any other character.

Evolution

• Undergo gradual changes for living organisms in response to change in the environment since the start of life.
• Theory of evolution gradually came to be accepted in the decades -
  • following Charles Darwin’s publications in “The Origin of Species” in 1858 and he assembled a great mass of evidence in support of the theory.

Lamarck’s Theory of Evolution

• Jean Baptist Lamarck (French biologist) put forward the theory of evolution in 1801.
• Theories supporting Jean Baptist Lamarck:
  • Theory of Need: Development of a new part or organ by an organism (plant or animal) comes from the need for that part or organ.
    • Snakes ancestors had short bodies and legs. As the land was changing forming narrow places and dense vegetation for snakes to walk through narrow places and dense vegetation, they started stretching bodies to crawl more easily.
    • Organs that become well developed, become functionless or disappear with disuse. -Each generation of snakes continues to stretch their bodies resulting in the strengthening of these bodies and their legs then became useless and disappeared being a hinderance from crawling through narrow places and dense vegetation.
  • Theory of Acquired Characteristics of Inheritance:
    • Structures or variation acquired by organism during their life time are transmitted to the next generation by inheritance.
    • The modern giraffe believed to be from the original giraffe with short neck and short legs.
    • When there was competition for low grasses among herbivorous animals the short neck and short legged giraffe started to starch their neck and legs to reach top vegetation:
• In need ducks developed webs between their toes for swimming.
• Theory of Natural Selection Darwin’s which propounded organic devolution by natural selection leading to the origin of species;
  • All living organisms should be allowed to produce more offspring than survive. -Struggle for existence occurs because of overpopulation of offspring with a competition for existence. -Offspring will show variation and no two individual are alike. -Adaptive characteristics are when some offsprings/individuals are adapted better for to survive the competition. -Inheritance of adaptive characteristics occur when the well adapted/fittest will transmit such variation to offspring that will cause development of new species through accumulation of favorable variation - a long time diverge (spread) from original stock.

Genetic Terminology

• Gene: Unit of inheritance transmitted from generation -responsible of controlling new traits.
• Chromosomes: Genetic material strand recognizable during cell division in nucleus.
  • Contain DNA (deoxyribonucleic acid) + Protein
• Character or Trait: Inheritable attributes or features like height.
• Phenotype: Observable features such as height, weight, skin colour.
• Genotype: Traits or sum of genes inherited from parents which makes the genetic makeup or constitution of an individuals which includes both dominant and recessive traits.
• Dominant Character: Trait expressed in offspring of two individuals with contrasting characters.
• Recessive Character: Trait from parent masked or not produced with dominant gene.
• Homozygous: Individual with identical alleles for a trait (TT or tt).
• Heterozygous: Individual with two different alleles for a trait (Tt or Rr).
• Hybrid: Offspring from generic different parents but of same species.
• Locus: Location of gene in the chromosome'
• Haploid: When an organism has one set of chromosomes in the reproductive cell (n).
• Diploid: When an organism has two sets of chromosomes in the body cell or animal/plant (2n)..
• Mutation: a change in the genetic makeup of an organism that results in a new characteristic that is inheritable.